The present invention relates generally to adjusting the air/fuel ratio in the cylinders of an internal combustion engine to control automotive emissions. More particularly, the present invention relates to a method and system for controlling an engine based on predicted operating conditions to maintain maximum catalytic activity.
To minimize the amount of emissions exhausted into the atmosphere, modern automotive vehicles generally include one or more catalytic converters, or emission control devices, in the exhaust system of the vehicle. These emission control devices store oxygen and NOx (collectively, oxidants) from the vehicle exhaust stream when the engine is operated with a relatively lean air/fuel ratio. On the other hand, when the engine is operated with a relatively rich air/fuel ratio, they release the stored oxygen and NOx, which then react with the HC and CO produced by the engine. In this way, the emission of both NOx and hydrocarbons (HC and CO) into the atmosphere is minimized.
The inventors have recognized a disadvantage with conventional air-fuel ratio control systems. In particular, the inventors have recognized that these systems attempt to maintain the engine at stoichiometry (or another desired air-fuel ratio). However, this has the disadvantage that engine air-fuel control is decoupled from the state of oxidant storage of the emission control device. The conventional system relies on air-fuel feedback to compensate for this oversight.
The inventors herein have further recognized that these known methods of adjusting cylinder air/fuel ratios, while effective, can be improved. In particular, the inventors have recognized that conventional air-fuel ratio control strategies do not take advantage of information that indicates whether future operating conditions will likely lead to depletion of oxidants in the emission control device. Nor do they take advantage of information that indicates whether future operating conditions will likely lead to saturation of oxidants in the emission control device. I.e., depletion of stored oxidants may lead to reductant breakthrough, and saturation of oxidants may lead to NOx breakthrough, thereby degrading emission control.
The above disadvantage is overcome by a method for controlling an engine coupled to an emission control device. The method comprises:
predicting whether future vehicle operating conditions may result in either a rich or lean excursion;
in response to said prediction, adjusting a set-point amount of oxidant stored in the emission control device to prevent said rich or lean excursion from causing an actual amount of stored oxidants to fall outside a predetermined range; and
adjusting a fuel injection amount into the engine based on said adjusted set-point.
By adjusting fuel injection based on a prediction of future operating conditions, it is possible to pre-set the catalyst to a condition that is better able to handle any excursion of the air-fuel ratio away from stoichiometry. In other words, if a lean air-fuel excursion is predicted, the oxidant level set-point of a catalyst can be set lower, thereby minimizing the chance of oxidant saturation in the catalyst. Further, if a rich air-fuel excursion is predicted, the oxidant level set-point can be set high, thereby minimizing the chance of oxidant depletion in the catalyst.
In another aspect of the present invention, the method comprises:
predicting whether future vehicle operating conditions may result in a change of an oxidant to reductant ratio of the exhaust gas;
in response to said prediction, adjusting a set-point amount of oxidant stored in the emission control device to prevent said change from causing an actual amount of stored oxidants to fall outside a predetermined range; and
adjusting an engine air-fuel ratio fed to the engine based on said adjusted set-point.
By adjusting fuel injection based on a prediction of future operating conditions, it is possible to pre-set the catalyst to a condition that is better able to handle any excursion of the air-fuel ratio away from stoichiometry. In other words, if a lean air-fuel excursion is predicted, the oxidant level set-point of a catalyst can be set lower, thereby minimizing the chance of oxidant saturation in the catalyst. Further, if a rich air-fuel excursion is predicted, the oxidant level set-point can be set high, thereby minimizing the chance of oxidant depletion in the catalyst.
An advantage of the above aspect of the invention is improved overall catalyst efficiency and reduced emissions.
Also note, there are various methods that can be used to predict whether future vehicle operating conditions may result in either a rich or lean excursion, or whether future operating conditions may result in a change of an oxidant to reductant ratio of the exhaust gas. For example, this prediction can be based on the operator pedal, engine operating conditions, statistical data, engine or catalyst temperature, throttle position, engine air-fuel ratio, or various other parameters. Further, it can be determined a priori and pre-programmed into the engine controller, or estimated on-line during vehicle operation.